Modified polyvinylidene fluoride, preparation method thereof, binder, positive pole piece and secondary battery

Abstract

The invention relates to modified polyvinylidene fluoride and a preparation method thereof, a binder, a positive pole piece and a secondary battery. The modified polyvinylidene fluoride comprises a polyvinylidene fluoride chain segment and a nitrogen-containing chain segment connected to one end of a main chain of the polyvinylidene fluoride chain segment. According to the invention, a nitrogen-containing chain segment with metal ion capturing and stabilizing effects is introduced to a main chain structure of polyvinylidene fluoride, so that the capability of adsorbing and stabilizing metal ions is improved. Meanwhile, by adding the modified polyvinylidene fluoride provided by the invention into a positive pole piece, the binding capacity to metal ions can be improved, the self-discharge of the battery is reduced, and the charge-discharge cycle stability of the battery is improved.

Description

Technical Field

[0001] The present invention belongs to the field of polymers and lithium-ion batteries, and in particular relates to a modified polyvinylidene fluoride and a preparation method thereof, a binder, a positive electrode sheet and a secondary battery. Background Art

[0002] Metal impurities can seriously harm the normal operation of lithium-ion batteries. During the charge and discharge process, free metal ions diffuse within the battery and migrate with the electrolyte to the negative electrode, where they deposit. This can clog the lithium-ion transport pathway, leading to reduced rate performance and increased self-discharge. Deposited metal can also hinder lithium ion embedding in graphite, causing capacity decay. In severe cases, it can even pierce the separator, causing a short circuit and safety issues. Furthermore, metal ions can catalyze various side reactions within the battery system, causing gassing and reduced cycle performance.

[0003] The metal impurities in lithium-ion batteries mainly come from the following processes. On the one hand, the battery raw materials and production processes easily introduce metal foreign matter, which is oxidized during the battery charging and discharging process and diffuses into the electrolyte. On the other hand, during the cycle life of the lithium-ion battery, the crystal structure of the positive electrode material (such as ternary nickel, cobalt and manganese) collapses, causing metal dissolution, resulting in the release of metal ions into the electrolyte and diffusion to the negative electrode side, thereby affecting the battery performance.

[0004] Currently, foreign matter in lithium-ion batteries is primarily controlled through raw materials and the production process. For example, this involves testing the foreign matter content of raw materials, adding demagnetization devices to production lines, prohibiting the use of iron tools on production lines, and replacing iron, copper, and zinc parts with stainless steel. However, these measures also have some shortcomings. Raw material testing is expensive, non-magnetic foreign matter such as copper, zinc, and aluminum cannot be intercepted, and production line improvements are costly. Furthermore, while strengthening raw material and production control measures can improve the content of metallic foreign matter in lithium-ion batteries to a certain extent, there is no effective way to address the dissolution of metal from the positive electrode material during battery cycling. Summary of the Invention

[0005] The object of the present invention is to provide a modified polyvinylidene fluoride, which can capture and stabilize metal ions in lithium-ion batteries, reduce the self-discharge of the batteries, and improve the charge and discharge cycle stability of the batteries.

[0006] In order to achieve the above object, the first aspect of the present invention provides a modified polyvinylidene fluoride, which comprises a polyvinylidene fluoride segment and a nitrogen-containing segment connected to one end of the main chain of the polyvinylidene fluoride segment; the nitrogen-containing segment comprises a structural unit represented by the following formula (I) or formula (II); ; wherein R1, R2 and R3 are the same or different and are each independently selected from one or more of H, substituted or unsubstituted 5-alkyl groups; R4 is selected from one of cyano and amide groups, or is selected from an alkyl group having a terminal cyano or amide group; R5 and R6 are the same or different and are each independently selected from one or more of H, carboxyl, hydroxyl and haloalkyl groups; a and b are each independently selected from any integer between 0 and 6.

[0007] Optionally, R5 and R6 are each independently selected from one or more of carboxyl, hydroxyl and halogenated alkyl; a and b are each independently selected from any integer between 0-6.

[0008] Optionally, based on the weight of the polyvinylidene fluoride segment, the content of the nitrogen-containing segment in the modified polyvinylidene fluoride is 1-10 wt%.

[0009] Optionally, based on the weight of the polyvinylidene fluoride segment, the content of the nitrogen-containing segment in the modified polyvinylidene fluoride is 2-6 wt %.

[0010] Optionally, the weight average molecular weight of the modified polyvinylidene fluoride is 20w-100w.

[0011] Optionally, the weight average molecular weight of the modified polyvinylidene fluoride is 40w-70w.

[0012] Optionally, the melting point of the modified polyvinylidene fluoride is 135-165° C.; and / or the crystallinity of the modified polyvinylidene fluoride is 32-50%; and / or the tensile strength of the modified polyvinylidene fluoride film is 30-50 MPa.

[0013] Optionally, the nitrogen-containing segment has any structural unit between the following formulas (1-1) to (1-10): .

[0014] A second aspect of the present invention provides a method for preparing modified polyvinylidene fluoride, the method comprising the following steps: The polyvinylidene fluoride raw material is subjected to an alkali treatment to cause an elimination reaction of the main chain of the polyvinylidene fluoride to obtain polyvinylidene fluoride having a terminal carbon-carbon double bond; Under polymerization reaction conditions, the polyvinylidene fluoride having a terminal carbon-carbon double bond and a nitrogen-containing monomer are polymerized; the nitrogen-containing monomer comprises a structure shown in the following formula (III) or formula (IV); ; wherein R1, R2 and R3 are the same or different and are each independently selected from one or more of H, substituted or unsubstituted alkyl groups; R4 is selected from one of cyano and amide groups, or selected from alkyl groups having cyano and amide terminal groups; R5 and R6 are the same or different and are each independently selected from one or more of H, carboxyl, hydroxyl and halogenated alkyl groups; and a and b are each independently selected from any integer between 0 and 6.

[0015] Optionally, R5 and R6 are each independently selected from one or more of carboxyl, hydroxyl and halogenated alkyl; a and b are each independently selected from any integer between 0-2.

[0016] Optionally, the weight average molecular weight of the polyvinylidene fluoride raw material is 1,000-100,000.

[0017] Optionally, the polyvinylidene fluoride raw material is subjected to an alkali treatment at a pH greater than or equal to 10; Optionally, the conditions for the alkali treatment include: a reaction temperature of 40-70° C.; and a reaction time of 4-8 hours.

[0018] Optionally, the mass ratio of the polyvinylidene fluoride raw material to the nitrogen-containing monomer is 100:(2-6.5).

[0019] Optionally, the polymerization reaction is carried out in the presence of an initiator and a chain transfer agent; the initiator is selected from one or more of benzoyl peroxide and azobisisobutyronitrile; and / or the chain transfer agent is selected from one or more of ethanethiol and isopropyl alcohol.

[0020] The third aspect of the present invention provides modified polyvinylidene fluoride prepared by the method of the second aspect of the present invention.

[0021] The fourth aspect of the present invention provides a binder, which includes any one of the modified polyvinylidene fluoride provided by the first aspect of the present invention.

[0022] The fifth aspect of the present invention provides a positive electrode plate, which includes a positive electrode collector and a positive electrode active material layer arranged on at least one surface of the positive electrode collector, and the positive electrode active material layer includes a positive electrode active material and any one of the modified polyvinylidene fluoride provided by the third aspect of the present invention.

[0023] Optionally, based on the weight of the positive electrode active material, the amount of the modified polyvinylidene fluoride is 1-10 wt %.

[0024] A sixth aspect of the present invention provides a secondary battery, comprising any one of the positive electrode sheets provided in the fifth aspect of the present invention.

[0025] A seventh aspect of the present invention provides an electrical device, which includes the secondary battery provided by the sixth aspect of the present invention.

[0026] Through the above technical solution, the modified polyvinylidene fluoride provided by the present invention has nitrogen-containing segments attached to its main chain structure. The functional groups in these nitrogen-containing segments have the function of capturing and stabilizing metal ions, and are evenly loaded onto the polyvinylidene fluoride structure, which helps improve the ability to adsorb and stabilize metal ions. At the same time, the modified polyvinylidene fluoride of the present invention, when added to the positive electrode sheet, can improve the ability to bind metal ions, reduce battery self-discharge, and improve the battery's charge and discharge cycle stability.

[0027] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the following detailed description, they are used to explain the present invention but do not constitute a limitation of the present invention. In the accompanying drawings: Figure 1 is an infrared spectrum of PVDF having a terminal carbon-carbon double bond in some exemplary embodiments of the present invention. DETAILED DESCRIPTION

[0029] The following is a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0030] In the prior art, polyvinylidene fluoride (PVDF) is often used as a binder to bond the active material particles in the electrode plate to the current collector and to form a coating on the active material particles. Based on the problem that the prior art cannot effectively control the dissolution of metal foreign matter from the positive electrode plate during battery cycling, the first aspect of the present invention provides a modified polyvinylidene fluoride, which includes a polyvinylidene fluoride segment and a nitrogen-containing segment connected to one end of the main chain of the polyvinylidene fluoride segment; the nitrogen-containing segment includes a structural unit represented by the following formula (I) or formula (II): ; wherein R1, R2 and R3 are the same or different and are each independently selected from one or more of H, substituted or unsubstituted alkyl groups; R4 is selected from one of cyano and amide groups, or is selected from an alkyl group having a terminal cyano or amide group; R5 and R6 are the same or different and are each independently selected from one or more of H, carboxyl, hydroxyl and haloalkyl groups; and a and b are each independently selected from any integer between 0 and 6.

[0031] In the above technical solution, a chain segment containing a functional group is introduced into the main chain structure of the modified polyvinylidene fluoride. The nitrogen atom in the functional group contains a lone pair of electrons, which can attack free metal ions with electron holes and form coordination bonds, thereby giving the modified PVDF the function of capturing and stabilizing metal ions.

[0032] In the nitrogen-containing segment structure represented by formula (I), the functional group R4 can be one of a cyano group and an amide group. The nitrogen atom in these groups contains a lone pair of electrons and can coordinate with metal ions to capture and stabilize metallic foreign matter. The functional group R4 can also be an alkyl group having a terminal cyano group or an amide group, and the nitrogen atom in the cyano group or the amide group is used to capture and stabilize metal ions. The length of the alkyl group is not limited. For example, the number of carbon atoms in the alkyl group can be any integer between 1 and 10, or any integer between 1 and 6.

[0033] In some embodiments of the present invention, the side chain of the nitrogen-containing segment represented by the above formula (II) is connected to an amino group or a substituted amino group, and the N atom in the amino group can form a coordination bond with a metal ion.

[0034] The polyvinylidene fluoride segments of the present invention contain F atoms. The electronegativity of the F atoms can attract positively charged metal ions, and together with the functional groups of the nitrogen-containing segments, they stabilize the metal ions. Furthermore, when R1, R2, and R3 in the nitrogen-containing segments are each independently selected from an alkyl group, a substituted or unsubstituted alkyl group may be attached to the structural unit. The number of carbon atoms in the alkyl group is not limited, and for example, an alkyl group having 1 to 5 carbon atoms may be used.

[0035] In addition, the N atom in the nitrogen-containing segment may be connected to an electronegative group, for example, one or more of a carboxyl group, a hydroxyl group, and a halogenated alkyl group (such as a fluorine-, chlorine-, or bromine-substituted alkyl group). The electronegativity of these groups is attracted to the positive charge of the metal ion to further stabilize the metal ion, thereby further improving the stabilization ability of the modified polyvinylidene fluoride for free metal ions.

[0036] In some preferred embodiments of the present invention, in the structural unit of the nitrogen-containing segment shown in above-mentioned formula (II), functional group is preferably amino compound;Electronegative group preferably has carboxyl, hydroxyl and haloalkyl with stronger electronegativity, and R5 and R6 are each independently selected from one or more of carboxyl, hydroxyl and haloalkyl. Wherein, a and b are each independently selected from 0, 1, 2, 3, 4, 5 or 6, and the number of the methylene radicals connected to N atoms is preferably any integer between 0-2, to avoid the distance between amino group and electronegative group on side chain group being too long to effectively assist in stabilizing metal ions. In order to take into account the mechanical properties of modified polyvinylidene fluoride and the performance of capturing and stabilizing metal foreign matter, the number of the methylene radicals connected to N atoms is more preferably any integer between 1-2.

[0037] In some embodiments of the present invention, based on the weight of the polyvinylidene fluoride segment, the content of the nitrogen-containing segment in the modified polyvinylidene fluoride can be 1-10 wt%, for example, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, or any numerical value within the aforementioned range, or a range consisting of any numerical value within the aforementioned range. By controlling the content of the nitrogen-containing segment in the modified polyvinylidene fluoride within a suitable range, the modified polyvinylidene fluoride has excellent structural properties, can effectively capture and stabilize metal ions, while reducing the influence of the nitrogen-containing segment on the mechanical properties of the modified polyvinylidene fluoride, so that the modified polyvinylidene fluoride has good bonding effect.

[0038] In some embodiments of the present invention, in order to further reduce the influence of the nitrogen-containing segment on the mechanical properties of the modified polyvinylidene fluoride and at the same time improve the stabilization effect of the modified polyvinylidene fluoride on metal ions, the content of the nitrogen-containing segment in the modified polyvinylidene fluoride is 2-6 wt% based on the weight of the polyvinylidene fluoride segment.

[0039] In some embodiments of the present invention, the weight average molecular weight of the modified polyvinylidene fluoride is 20w-100w, for example, it can be 20w, 30w, 40w, 50w, 55w, 59w, 62w, 70w, 80w, 90w, 100w, or any value within the above range, or a range consisting of any values ​​within the above range. By controlling the weight average molecular weight of the modified polyvinylidene fluoride within a suitable range, it is beneficial to obtain a modified polyvinylidene fluoride with a suitable nitrogen-containing segment content, thereby achieving effective capture and stabilization of metal foreign matter. If the molecular weight of the modified polyvinylidene fluoride is too large, it will be difficult to dissolve as a binder, and it will be prone to agglomeration, and it will also increase the viscosity of the slurry; if the weight average molecular weight of the nitrogen-containing segment is too small, the viscosity of the slurry will be too small, which is not conducive to the formation of a stable pole piece.

[0040] In some embodiments of the present invention, in order to further control the stabilizing effect of the modified polyvinylidene fluoride on metal ions, the weight average molecular weight of the modified polyvinylidene fluoride is 40w-70w.

[0041] In some embodiments of the present invention, the modified polyvinylidene fluoride has a melting point of 135-165°C, a crystallinity of 32-50%, and / or a tensile strength of 30-50 MPa. The modified polyvinylidene fluoride provided by the present invention has mechanical properties similar to those of conventional PVDF, making it suitable for use in the preparation of lithium-ion batteries.

[0042] In some preferred embodiments of the present invention, the melting point of the prepared polyvinylidene fluoride is 155-165° C.; and / or the crystallinity of the prepared polyvinylidene fluoride is 37-50%; and / or the tensile strength of the prepared polyvinylidene fluoride film is 35-50 MPa.

[0043] In some specific embodiments of the present invention, the nitrogen-containing segment has any structural unit between the following formulas (1-1) to (1-10): .

[0044] A second aspect of the present invention provides a method for preparing modified polyvinylidene fluoride, the method comprising the following steps: The polyvinylidene fluoride raw material is subjected to an alkali treatment to cause an elimination reaction of the main chain of the polyvinylidene fluoride to obtain polyvinylidene fluoride having a terminal carbon-carbon double bond; Under polymerization reaction conditions, the polyvinylidene fluoride having a terminal carbon-carbon double bond and a nitrogen-containing monomer are polymerized; the nitrogen-containing monomer comprises a structure shown in the following formula (III) or formula (IV); ; wherein R1, R2 and R3 are the same or different and are each independently selected from one or more of H, substituted or unsubstituted alkyl groups; R4 is selected from one of cyano and amide groups, or is selected from an alkyl group having a terminal cyano or amide group; R5 and R6 are the same or different and are each independently selected from one or more of H, carboxyl, hydroxyl and haloalkyl groups; and a and b are each independently selected from any integer between 0 and 6.

[0045] The preparation method of the modified polyvinylidene fluoride of the present invention can directly utilize polyvinylidene fluoride purchased on the market to carry out activation reaction and introduce functional segments.

[0046] In the above preparation method, during the alkali treatment process, under the action of the alkaline reagent, the F atoms at the ends of the polyvinylidene fluoride molecules are removed by forming HF and undergoing an elimination reaction, thereby forming polyvinylidene fluoride molecules containing terminal carbon-carbon double bonds, that is, PVDF with terminal carbon-carbon double bonds. The terminal carbon-carbon double bonds of the alkali-treated PVDF are then polymerized with nitrogen-containing monomers, thereby introducing modified functional groups into the PVDF backbone molecular chain.

[0047] In some embodiments of the present invention, R5 and R6 are each independently selected from one or more of carboxyl, hydroxyl, and haloalkyl. The haloalkyl may be a fluorine-, chlorine-, or bromine-substituted alkyl group, such as trifluoromethyl or fluoroethyl. The electronegativity of these groups attracts the positive charge of the metal ion, further stabilizing the metal ion. In the above structure, a and b are each independently selected from 0, 1, or 2, preferably 1 or 2, thereby enhancing the metal ion stabilization effect of the functional group.

[0048] In some embodiments of the present invention, the molecular weight of the polyvinylidene fluoride raw material can be 1000-100000, for example, the molecular weight of the polyvinylidene fluoride can be 1000, 10000, 20000, 30000, 40000, 50000, 60000, 70000, 80000, 90000, 100000 or any numerical value within the aforementioned range, or the range composed of any numerical value within the aforementioned range. When the molecular weight of the polyvinylidene fluoride raw material is small, more graft monomers are needed to act as "bridges" to connect. Due to the steric hindrance effect of the PVDF molecule, the ability of the modified polyvinylidene fluoride to adsorb metal ions is slightly weakened, and the crystallinity and tensile strength decrease. However, when the molecular weight of the polyvinylidene fluoride raw material is large, there are few graft monomers acting as "bridges" to connect, and the ability to adsorb metal ions is slightly enhanced. The graft monomers form short chains, so the crystallinity of the modified polyvinylidene fluoride is higher, but the tensile strength decreases.

[0049] In some preferred embodiments, the molecular weight of the polyvinylidene fluoride raw material is 2w-7w. The use of polyvinylidene fluoride with a higher molecular weight is beneficial to retaining the crystallinity and mechanical properties of polyvinylidene fluoride, reducing the influence of grafting modification on the mechanical properties of modified polyvinylidene fluoride, thereby avoiding affecting its performance as a binder. On the other hand, due to the steric hindrance effect of the molecular chain, it is more conducive to the formation of terminal carbon-carbon double bonds.

[0050] In the present invention, the alkali treatment of the polyvinylidene fluoride raw material needs to be performed under an alkaline reagent of appropriate concentration. In some embodiments, the alkali treatment of the polyvinylidene fluoride raw material is performed under a strong alkaline condition of pH greater than or equal to 10.

[0051] In some embodiments of the present invention, PVDF is dissolved as a precursor material in a polar organic solvent to open the PVDF molecular chains, and then an alkaline reagent is added for treatment. The polar organic solvent used to dissolve the PVDF precursor material can be common solvents such as dimethyl sulfoxide (DMSO) and N-methylpyrrolidone (NMP). The alkaline reagent used for the alkaline treatment can be selected from alcoholic solutions of one or more of sodium hydroxide, potassium hydroxide, and tetramethylammonium hydroxide. For example, in some specific embodiments, the alkaline reagent is an ethanolic solution of sodium hydroxide or potassium hydroxide.

[0052] In some embodiments of the present invention, the alkali treatment conditions include a reaction temperature of 40-70°C and a reaction time of 4-8 hours. When the alkali treatment is performed on polyvinylidene fluoride under relatively mild conditions, the terminal groups of the polyvinylidene fluoride molecular chain, which have less steric hindrance, preferentially react to form terminal carbon-carbon double bonds. If the reaction conditions are more severe, a large number of disordered double bonds may form within the polyvinylidene fluoride molecular chain.

[0053] In some specific embodiments of the present invention, the nitrogen-containing monomer is selected from compounds having the following structures: CH2=CHCN, CH2=CHNHCH2CH2OH, CH2=CHNH2, CH2=CHCONH2, CH2=CHCH2CN, CH2=CH-N(CH2COOH)2, CH2=CH-N(CH2CH2COOH)2, CH2=CH-N(CH2CH2COOH)2.

[0054] In some embodiments of the present invention, the mass ratio of the polyvinylidene fluoride raw material to the nitrogen-containing monomer is 100:(2-6.5) to control the weight average molecular weight of the modified polyvinylidene fluoride within an appropriate range.

[0055] In some embodiments of the present invention, the polymerization reaction is carried out in the presence of an initiator and a chain transfer agent. The initiator is selected to be soluble in an organic solvent and can be selected from one or more of benzoyl peroxide and azobisisobutyronitrile. The chain transfer agent is used to adjust the molecular weight of the modified PVDF (i.e., modified polyvinylidene fluoride) and can be selected from one or more of ethanethiol and isopropanol. In some specific preferred embodiments of the present invention, the polymerization reaction conditions include: a reaction temperature of 50-80°C, preferably 60-75°C; a time for adding the initiator dropwise during the initiation phase of 0-4 hours, preferably 1-2 hours; and a heat-insulating reaction time of 0-6 hours, preferably 3-4 hours. By controlling the polymerization reaction conditions, modified PVDF with a suitable molecular weight can be obtained.

[0056] In some embodiments of the present invention, the polymerization reaction is carried out in an inert organic solvent, which can be selected from one or more of N-methylpyrrolidone (NMP), toluene, ethyl acetate and butanone.

[0057] In some embodiments of the present invention, the method further comprises: adjusting the pH of the reaction system after the alkali treatment to 6-9.

[0058] The third aspect of the present invention provides a modified polyvinylidene fluoride prepared by the above method.

[0059] A fourth aspect of the present invention provides a binder comprising the modified polyvinylidene fluoride provided in the first aspect of the present invention or the modified polyvinylidene fluoride provided in the third aspect of the present invention. The modified polyvinylidene fluoride provided in the present invention has a nitrogen-containing segment introduced into its skeleton structure. The nitrogen atoms in the nitrogen-containing segment contain lone pairs of electrons, which can attack free metal ions with electron holes and form coordination bonds, thereby capturing and stabilizing metallic foreign matter.

[0060] A fifth aspect of the present invention provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises a positive electrode active material and the aforementioned binder. The binder provided by the present invention comprises the modified polyvinylidene fluoride of the first aspect of the present invention, and the use of the modified polyvinylidene fluoride is beneficial for improving battery performance.

[0061] In some embodiments of the present invention, the amount of modified polyvinylidene fluoride is 1-10 wt% based on the weight of the positive electrode active material. By controlling the amount of modified polyvinylidene fluoride within an appropriate range, it is beneficial to obtain a positive electrode slurry with an appropriate viscosity and to capture and stabilize metallic foreign matter. Because excessive use of a binder can reduce the content of the positive electrode active material, the amount of modified polyvinylidene fluoride is preferably 1-5 wt%.

[0062] In some embodiments of the present invention, the positive electrode active material is selected from one or more of lithium cobalt oxide, lithium nickel manganese cobalt oxide, lithium nickel manganese aluminum oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate and lithium titanate.

[0063] In some embodiments of the present invention, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil or other metal foil may be used. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, or silver alloy, etc.) on a polymer substrate (such as polypropylene, polyethylene, etc.).

[0064] A sixth aspect of the present invention provides a secondary battery, comprising the positive electrode sheet according to the fourth aspect of the present invention.

[0065] The secondary battery comprises a positive electrode sheet, a separator and a negative electrode sheet stacked in sequence.

[0066] In some embodiments of the present invention, the material of the diaphragm can be selected from one or more of high-density polyethylene, ultra-high-density polyethylene, low-density polyethylene, linear low-density polyethylene, high-density polypropylene, ultra-high-density polypropylene, polyimide and polyvinylidene fluoride.

[0067] In some embodiments of the present invention, the active material in the negative electrode plate can be selected from one or more of natural graphite, artificial graphite, carbon black, petroleum coke, acetylene black, Ketjen black, carbon fiber and polyphenylene derivatives.

[0068] The present invention also provides an electrical device, which includes the aforementioned secondary battery.

[0069] The electrical equipment may include mobile devices such as mobile phones and laptop computers, as well as electric vehicles, electric trains, and energy storage devices.

[0070] The present invention is further described in detail below by way of examples, but the present invention is not limited thereto. The raw materials used in the examples can be obtained through commercial routes.

[0071] The nitrogen-containing monomer CH2=CH-N(CH2COOH)2 in the embodiment can be prepared by an addition reaction between ethyleneamine (CH2=CH-NH2) and chloroacetic acid (ClCH2COOH), and can also be purchased commercially.

[0072] The molecular weight of the modified polyvinylidene fluoride in the examples and comparative examples of the present invention can be measured by gel permeation chromatography.

[0073] Example 1 This embodiment is used to illustrate the preparation method of modified polyvinylidene fluoride of the present invention, which includes the following steps: S1. PVDF powder with a weight-average molecular weight of 5 w is dissolved in N-methylpyrrolidone (NMP) to obtain a PVDF NMP solution; sodium hydroxide ethanol solution (the mass fraction of NaOH is 40 wt%) is added to the PVDF NMP solution, and the obtained mixed solution is heated to 60 ° C for alkali treatment for 6 hours to allow the PVDF molecules to undergo an elimination reaction to form PVDF with terminal carbon-carbon double bonds. After the reaction is completed, the pH of the reaction system is adjusted to 6-9 with hydrochloric acid. The weight ratio of PVDF powder, NMP and sodium hydroxide is 9:100:4; S2. Add PVDF with a terminal carbon-carbon double bond and a nitrogen-containing monomer with the structure of CH2=CH-N(CH2COOH)2 to N-methyl-2-pyrrolidone (NMP) and mix them evenly. Add initiator azobisisobutyronitrile (AIBN) and chain transfer agent isopropyl alcohol, raise the temperature to 65°C, and carry out free radical polymerization. The reaction time is 3 hours to obtain a modified polyvinylidene fluoride solution grafted with aminodiacetic acid; wherein the weight ratio of PVDF with a terminal carbon-carbon double bond, nitrogen-containing monomer CH2=CH-N(CH2COOH)2, AIBN and isopropyl alcohol is 100:2:0.02:0.01; and the weight average molecular weight of the modified polyvinylidene fluoride obtained is controlled to be about 59w; S3. Adjust the pH of the modified polyvinylidene fluoride solution grafted with aminodiacetic acid to 4.5 with hydrochloric acid, dry to remove moisture, then wash to remove residual small molecules, dry and crush to obtain a finished modified PVDF powder, recorded as PVDF-1.

[0074] Infrared test was performed on PVDF-1. Figure 1The infrared spectrum of PVDF-1 shown in the figure shows that at 1650 cm -1 An absorption peak appears near the surface, proving the presence of a small amount of terminal C=C.

[0075] Example 2 The method for preparing modified polyvinylidene fluoride in this embodiment is basically similar to that in Example 1, except that: The weight ratio of PVDF with a terminal carbon-carbon double bond, nitrogen-containing monomer CH2=CH-N(CH2COOH)2, AIBN, and isopropyl alcohol is 100:5:0.05:0.25. The resulting modified PVDF powder sample is designated PVDF-2.

[0076] Example 3 The method for preparing modified polyvinylidene fluoride in this embodiment is basically similar to that in Example 1, except that: The weight ratio of PVDF with a terminal carbon-carbon double bond, nitrogen-containing monomer CH2=CH-N(CH2COOH)2, AIBN, and isopropyl alcohol is 100:8:0.08:0.04. The resulting modified PVDF powder sample is designated PVDF-3.

[0077] Example 4 The method for preparing modified polyvinylidene fluoride in this embodiment is basically similar to that in Example 1, except that: The weight ratio of PVDF with a terminal carbon-carbon double bond, nitrogen-containing monomer CH2=CH-N(CH2COOH)2, AIBN, and isopropyl alcohol is 100:12:0.12:0.06. The resulting modified PVDF powder sample is designated PVDF-4.

[0078] Example 5 The method for preparing modified polyvinylidene fluoride in this embodiment is basically similar to that in Example 1, except that: In step S1, the weight average molecular weight of the PVDF powder is 1w; in step S2, the polymerization reaction time is appropriately extended to 6h to control the weight average molecular weight of the finished modified PVDF to be approximately 59w; The obtained modified PVDF powder sample was recorded as PVDF-5.

[0079] Example 6 The method for preparing modified polyvinylidene fluoride in this embodiment is basically similar to that in Example 1, except that: In step S1, the weight average molecular weight of the PVDF powder is 2w; in step S2, the polymerization reaction time is appropriately extended to 5h to control the weight average molecular weight of the finished modified PVDF to be approximately 59w; The obtained modified PVDF powder sample was recorded as PVDF-6.

[0080] Example 7 The method for preparing modified polyvinylidene fluoride in this embodiment is basically similar to that in Example 1, except that: In step S1, the weight average molecular weight of PVDF is 7W; in step S2, the polymerization reaction time is shortened to 2.7 hours to control the weight average molecular weight of the finished modified PVDF to be approximately 60W; The obtained modified PVDF powder sample was recorded as PVDF-7.

[0081] Example 8 The method for preparing modified polyvinylidene fluoride in this embodiment is basically similar to that in Example 1, except that: In step S1, the weight average molecular weight of the PVDF powder is 10 W; in step S2, the polymerization reaction time is shortened to 2.3 hours to control the weight average molecular weight of the finished modified PVDF to be approximately 59 W; The obtained modified PVDF powder sample was recorded as PVDF-8.

[0082] Example 9 The method for preparing modified polyvinylidene fluoride in this embodiment is basically similar to that in Example 1, except that: In step S2, the structure of the nitrogen-containing monomer is CH2=CH-N(CH2CH2CH2COOH)2; the obtained modified PVDF powder sample is recorded as PVDF-9.

[0083] Example 10 The method for preparing modified polyvinylidene fluoride in this embodiment is basically similar to that in Example 1, except that: In step S2, the structure of the nitrogen-containing monomer is acrylamide CH2=CHCONH2; the obtained modified PVDF powder sample is recorded as PVDF-10.

[0084] Example 11 The method for preparing modified polyvinylidene fluoride in this embodiment is basically similar to that in Example 1, except that: In step S2, the structure of the nitrogen-containing monomer is ethyleneamine CH2=CHNH2; the obtained modified PVDF powder sample is recorded as PVDF-11.

[0085] Example 12 The method for preparing modified polyvinylidene fluoride in this embodiment is basically similar to that in Example 1, except that: In step S2, the structure of the nitrogen-containing monomer is acrylonitrile CH2=CHCN; the obtained modified PVDF powder sample is recorded as PVDF-12.

[0086] Comparative Example 1 Conventional unmodified PVDF was used as comparative example 1.

[0087] The properties of the modified PVDF-1 to PVDF-12 prepared in Examples 1-12 and the PVDF in Comparative Example 1 are shown in Table 1.

[0088] Table 1

[0089] It can be seen from the above table that the mechanical properties of the modified polyvinylidene fluoride obtained in the present invention are comparable to those of unmodified PVDF.

[0090] Among them, when the molecular weight of the PVDF raw material is too small, the crystallinity of the polyvinylidene fluoride chain segment in the modified PVDF is low, resulting in low tensile strength; when the molecular weight of the PVDF raw material increases, the crystallinity and tensile strength of the modified PVDF are improved; however, when the molecular weight of the PVDF raw material is too large, the nitrogen-containing monomers themselves polymerize into long chains, the flexibility increases, and the overall tensile strength of the modified PVDF is reduced.

[0091] Performance Testing Positive electrodes were prepared using the modified polyvinylidene fluoride PVDF-1 to PVDF-12 prepared in Examples 1-12 as binders, and the unmodified conventional binder PVDF in Comparative Example 1 was used to prepare positive electrodes. Positive electrodes containing lithium ferrous phosphate as the positive electrode active material were prepared, and lithium iron phosphate batteries were prepared using the prepared positive electrodes, negative electrodes, and separators, which were designated B-1 to B-12 and DB-1, respectively.

[0092] Preparation of positive electrode sheet: The positive electrode active material lithium iron phosphate, conductive agent carbon black, binder and N-methylpyrrolidone (NMP) were stirred and mixed in a weight ratio of 100:2:2.5:69 to obtain a positive electrode slurry; the positive electrode slurry was evenly coated on both sides of a 12μm thick positive electrode current collector aluminum foil, with a single-sided coating layer density of 190g / m 2 , forming a positive electrode active material layer, which is then dried, cold pressed, and cut to obtain a positive electrode sheet.

[0093] Preparation of negative electrode sheet: Mix the negative electrode active material artificial graphite, the conductive agent acetylene black, the binder styrene-butadiene rubber, and the thickener sodium carboxymethyl cellulose in a weight ratio of 100:2:2.2:1.8, add deionized water, and stir to obtain the negative electrode slurry. The negative electrode slurry is evenly coated on an 8μm thick negative electrode current collector copper foil to form a negative electrode active material layer. After drying and roller pressing, the negative electrode is obtained.

[0094] Preparation of the battery: The positive electrode sheet, polypropylene separator and negative electrode sheet are stacked in sequence, then placed in the outer packaging foil aluminum-plastic film after winding and welding the tabs, and the above-mentioned electrolyte is injected. After vacuum packaging, standing, formation, shaping and capacity testing, a soft-pack lithium-ion battery is obtained.

[0095] Test Example 1 Metal element content test: The lithium-ion batteries B-1 to B-12 and DB-1 prepared above were subjected to charge and discharge tests. The charge and discharge test method includes the steps of placing the prepared lithium iron phosphate battery in a 60°C environment, performing a charge and discharge test at a 1C rate, and cycling 500 times.

[0096] After 500 charge-discharge cycles, the batteries were disassembled and the transition metal content of the positive and negative electrodes was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES). Transition metals include iron, nickel, copper, and zinc. The statistical results are shown in Table 2.

[0097] Table 2

[0098] It can be seen from the element content data in the above table that after 500 cycles of charge and discharge test, in the unmodified battery of Comparative Example 1, a certain amount of metal elements were detected in both the diaphragm and the negative electrode; while in the battery prepared with modified PVDF, the metal element content of the diaphragm and the negative electrode was significantly reduced, and the metal elements on the positive electrode were higher than those on the positive electrode in Comparative Example 1, indicating that the modified PVDF provided by the present invention can bind metal elements to the positive electrode.

[0099] Among them, when the molecular weight of the PVDF raw material is small, more grafted monomers are needed to act as "bridges" to connect. Due to the steric hindrance effect of the PVDF molecules, the nitrogen-containing monomers are hindered from playing a role, and the ability to adsorb metal ions is slightly weakened; when the molecular weight of the PVDF raw material is large, there are fewer nitrogen-containing monomers that act as "bridges" to connect, the ability to adsorb metal ions is slightly enhanced, and the nitrogen-containing monomers form short chains.

[0100] In Example 9, when the number of carbon atoms between the amino group and the carboxyl group is 3, the distance between the amino group and the electronegative group is too long, and the effect of assisting in stabilizing the metal ion is slightly reduced.

[0101] In Example 10, when the nitrogen-containing segment side chain is connected to amide, the metal ions are adsorbed only by the amide group, and the adsorption capacity is slightly reduced; in Example 11, when the amino group is directly connected to the main chain, it is blocked by the main chain and surrounding atoms, and the ability to adsorb metal ions is slightly weakened.

[0102] In Example 12, the cyano monomer has a stronger electronegativity and a stronger ability to attract complex metal ions.

[0103] Test Example 2 Battery self-discharge test: After 500 charge-discharge cycles, the battery was charged to 25% SOC. The initial voltage, V1, was recorded using a multimeter. The battery was then left at room temperature (25°C) for three days, and the voltage, V2, was measured again. The change in open-circuit voltage was calculated using the equation V = V1 - V2, and the battery's self-discharge performance was evaluated. The self-discharge test results are shown in Table 3.

[0104] Table 3

[0105] From the data in the above table, it can be found that compared with the conventional PVDF in Comparative Example 1, the self-discharge of the lithium ion battery prepared using the modified PVDF provided by the present invention as a binder is significantly reduced, the migration of metal ions in the battery is reduced, and the charge loss is small.

[0106] The preferred embodiments of the present disclosure are described in detail above. However, the present disclosure is not limited to the specific details of the above embodiments. Within the technical concept of the present disclosure, various simple modifications can be made to the technical solutions of the present disclosure, and these simple modifications all fall within the scope of protection of the present disclosure.

[0107] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present disclosure will not further describe various possible combinations.

[0108] In addition, the various embodiments of the present disclosure may be arbitrarily combined, and as long as they do not violate the concept of the present disclosure, they should also be regarded as the contents disclosed by the present disclosure.

Claims

1. A modified polyvinylidene fluoride, characterized in that: The modified polyvinylidene fluoride comprises a polyvinylidene fluoride segment and a nitrogen-containing segment connected to one end of the main chain of the polyvinylidene fluoride segment; the nitrogen-containing segment comprises a structure represented by the following formula (I) or formula (II); ； wherein R1, R2 and R3 are the same or different and are each independently selected from one or more of H, substituted or unsubstituted alkyl groups; R4 is selected from one of cyano and amide groups, or is selected from an alkyl group having a terminal cyano or amide group; R5 and R6 are the same or different and are each independently selected from one or more of H, carboxyl, hydroxyl and haloalkyl groups; and a and b are each independently selected from any integer between 0 and 6.

2. The modified polyvinylidene fluoride according to claim 1, wherein R5 and R6 are each independently selected from one or more of carboxyl, hydroxyl and halogenated alkyl; a and b are each independently selected from any integer between 0 and 2.

3. The modified polyvinylidene fluoride according to claim 1, wherein Based on the weight of the polyvinylidene fluoride segment, the content of the nitrogen-containing segment in the modified polyvinylidene fluoride is 1-10 wt%.

4. The modified polyvinylidene fluoride according to claim 3, wherein Based on the weight of the polyvinylidene fluoride segment, the content of the nitrogen-containing segment in the modified polyvinylidene fluoride is 2-6 wt%.

5. The modified polyvinylidene fluoride according to any one of claims 1 to 4, wherein The weight average molecular weight of the modified polyvinylidene fluoride is 20 w-100 w.

6. The modified polyvinylidene fluoride according to claim 5, wherein The weight average molecular weight of the modified polyvinylidene fluoride is 40 w-70 w.

7. The modified polyvinylidene fluoride according to claim 1, wherein The melting point of the modified polyvinylidene fluoride is 135-165°C; and / or The modified polyvinylidene fluoride has a crystallinity of 32-50%; and / or The modified polyvinylidene fluoride film has a tensile strength of 30-50 MPa.

8. The modified polyvinylidene fluoride according to claim 1, wherein The nitrogen-containing segment has any structural unit between the following formulas (1-1) to (1-10): 。 9. A method for preparing modified polyvinylidene fluoride, characterized in that: The method comprises the following steps: The polyvinylidene fluoride raw material is subjected to an alkali treatment to cause an elimination reaction of the main chain of the polyvinylidene fluoride to obtain polyvinylidene fluoride having a terminal carbon-carbon double bond; Under polymerization reaction conditions, the polyvinylidene fluoride having a terminal carbon-carbon double bond and a nitrogen-containing monomer are polymerized; the nitrogen-containing monomer comprises a structure shown in the following formula (III) or formula (IV); ； wherein R1, R2 and R3 are the same or different and are each independently selected from one or more of H, substituted or unsubstituted alkyl groups; R4 is selected from one of cyano and amide groups, or is selected from an alkyl group having a terminal cyano or amide group; R5 and R6 are the same or different and are each independently selected from one or more of H, carboxyl, hydroxyl and haloalkyl groups; and a and b are each independently selected from any integer between 0 and 6.

10. The method according to claim 9, wherein: R5 and R6 are each independently selected from one or more of carboxyl, hydroxyl and halogenated alkyl; a and b are each independently selected from any integer between 0 and 2.

11. The method according to claim 10, wherein: The weight average molecular weight of the polyvinylidene fluoride raw material is 1,000-100,000.

12. The method according to claim 9, wherein performing an alkali treatment on the polyvinylidene fluoride raw material at a pH greater than or equal to 10; Optionally, the conditions for the alkali treatment include: a reaction temperature of 40-70° C.; and a reaction time of 4-8 hours.

13. The method according to claim 9, wherein: The mass ratio of the polyvinylidene fluoride raw material to the nitrogen-containing monomer is 100:(2-6.5).

14. The method according to claim 9, wherein The polymerization reaction is carried out in the presence of an initiator and a chain transfer agent; The initiator is selected from one or more of benzoyl peroxide and azobisisobutyronitrile; and / or The chain transfer agent is selected from one or more of ethyl mercaptan and isopropyl alcohol.

15. Modified polyvinylidene fluoride prepared by the method according to any one of claims 9 to 14.

16. An adhesive, characterized in that: The binder comprises the modified polyvinylidene fluoride according to any one of claims 1 to 8 or claim 15.

17. A positive electrode plate, characterized in that: The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer includes a positive electrode active material and the binder according to claim 16 .

18. The positive electrode sheet according to claim 17, wherein: Based on the weight of the positive electrode active material, the amount of the modified polyvinylidene fluoride is 1-10 wt%.

19. A secondary battery, characterized in that: The secondary battery comprises the positive electrode sheet according to claim 17 or 18.

20. An electrical device, characterized in that: The electric device includes the secondary battery according to claim 19.

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